Oxide Barrier Layer Research Articles

Detection of Odontoglossum Ringspot Virus Infected Phalaenopsis Using a Nano-Structured Biosensor

In this study, a highly sensitive biosensor based on a 3D nano-structured sensing electrode made from an anodic aluminum oxide (AAO) barrier layer with the uniform distribution of gold nanoparticles (GNPs) is proposed for label-free detection of orchid viruses. This nanostructured electrode was utilized for developing a label-free sensing system based on electrochemical impedance spectroscopy (EIS) to directly detect the Odontoglossum ringspot virus (ORSV) concentration from orchid leaves. The ORSV antibody (anti-ORSV) was employed as a detecting probe and was covalently immobilized on the nanostructured sensing electrode surface through a self-assembled monolayer (SAM) method. Through a selective binding interaction between specific antigen and antibody pairs, our nanostructured biosensors can provide a high specificity and accuracy sensing result. Our experimental results revealed that the specific binding of ORSV onto the immobilized anti-ORSV gave rise to the incremental interfacial charge transfer resistance of the electrode, allowing one to distinguish between healthy and infected orchids in a fast and accurate manner. The linear detection range is 50,000 ng/mL–0.5 ng/mL with R2 = 0.9686. A detection limit of 0.345 ng/mL can be achieved. These results demonstrated that our ORSV biosensor is feasible for field applications.

Reduced graphene oxide as a water, carbon dioxide and oxygen barrier in plasticized poly(vinyl chloride) films.

Herein, we report the incorporation of a 10 μm thick reduced graphene oxide (RGO) barrier layer in a plasticized poly(vinyl chloride) (PVC) film as the main constituent in ion-selective membranes used in potentiometric solid-contact ion-selective electrodes (SCISE). Fourier transform infrared attenuated total reflection (FTIR-ATR) and oxygen transmission rate (OTR) measurements showed that the embedded RGO barrier efficiently impedes the diffusion of liquid water, carbon dioxide and oxygen (O2) through the 400 μm thick PVC film, which causes potential instability and irreproducibility of the SCISEs. The measurements revealed that the RGO layer completely blocks the carbon dioxide diffusion, while it fully blocks the water diffusion for 16 h and reduced the OTR by 85% on average. The μm-thick RGO films used in this study were easier to handle and incorporate into host polymers, and form more efficient and robust barriers compared to the mono-, few- and multilayer graphene commonly applied as barrier layers for liquids and gases. We also demonstrated that the FTIR-ATR technique employed in the permeability measurements is a versatile and very sensitive technique for studying the diffusion of small amounts of water and carbon dioxide through graphene-based thin films.

Morphology evolution of porous anodic alumina in mixed H3PO4/NH4F electrolytes

Most anodizations of Al to obtain porous anodic alumina (PAA) are conducted in acid solution including sulfuric acid, oxalic acid and phosphoric acid solutions. It is well known that fluoride-containing electrolyte is generally used to fabricate anodic TiO2 nanotubes (ATNTs). Nevertheless, there are few articles reporting anodization processes of Al in NH4F electrolyte and mixed H3PO4/NH4F/ethylene glycol electrolyte. Herein, in order to explore different morphology of anodic alumina film during a series of constant current anodization processes, NH4F solutions, H3PO4 solutions of different concentrations (2wt%, 4wt% and 6wt%), and their mixed electrolytes containing different concentrations of NH4F and H3PO4 were employed. It is the first time for Al to be performed anodization in NH4F solution under constant current. PAA with thick pore wall, thick barrier oxide layer and narrow pores were obtained. The results show that three stages of voltage-time curve are not the requirement of obtaining PAA. Which anodic oxide (porous anodic oxide or compact anodic oxide) occurs only depends on the size of the electronic current. As long as the electronic current is high enough, porous anodic oxide is easily obtained. While the concentration of NH4F increases to 0.6wt%, the barrier oxide is much thicker than that in H3PO4 electrolyte due to the high ionic current and low electronic current.

Improvement of Self-Heating of Indium Gallium Zinc Aluminum Oxide Thin-Film Transistors Using Al2O3 Barrier Layer

To study the self-heating effect, aluminum oxide (Al2O3) barrier layers of various thicknesses have been inserted between the channel layer and insulator layer in bottom-gate-type indium gallium zinc aluminum oxide (IGZAO) thin-film transistors (TFTs). Each IGZAO channel layer was deposited on indium tin oxide (ITO)-coated glass substrate by using a magnetron radiofrequency cosputtering system with dual targets composed of indium gallium zinc oxide (IGZO) and Al. The 3s orbital of Al cation provided an extra transport pathway and widened the conduction-band bottom, thus increasing the electron mobility of the IGZAO films. The Al–O bonds were able to sustain the oxygen stability of the IGZAO films. The self-heating behavior of the resulting IGZAO TFTs was studied by Hall measurements on the IGZAO films as well as the electrical performance of the IGZAO TFTs with Al2O3 barrier layers of various thicknesses at different temperatures. IGZAO TFTs with 50-nm-thick Al2O3 barrier layer were stressed by positive gate bias stress (PGBS, at gate–source voltage V GS = 5 V and drain–source voltage V DS = 0 V); at V GS = 5 V and V DS = 10 V, the threshold voltage shifts were 0.04 V and 0.2 V, respectively, much smaller than for the other IGZAO TFTs without Al2O3 barrier layer, which shifted by 0.2 V and 1.0 V when stressed under the same conditions.

Photoluminescence performance limits of Si nanocrystals in silicon oxynitride matrices

The present comprehensive study of photoluminescence (PL) quantum yield (QY) of Si nanocrystals (SiNCs) in Si-rich oxynitride (SRON) superlattices was performed over a broad set of samples. The PL QY is sensitive mostly to the thickness of SRON and barrier oxide layers and to the passivation procedures. Annealing in hydrogen improves the QY proportionally to the NC surface area by passivating the NC/oxide interface defects present at a surface density of about 2.5 × 1012 cm−2. The maximum external QY of nearly 30% is found in well-passivated superlattices with a SiNC size of about 4 nm and a SiO2 barrier thickness of 2 nm or larger. We reveal the existence of an extended near-infrared tail of the PL spectra, whose weak intensity anti-correlates with the external QY. The relative intensity of this emission increases with temperature as well as for strong excitation above the PL saturation level and may be related to excitation energy transfer to the structural defects near NCs. Finally, we discuss the possible mechanisms which are responsible for limiting the attainable PL QY and which may be the subject of future efforts to further increase the PL QY.

Design of durable and efficient poly(arylene ether nitrile)/bioinspired polydopamine coated graphene oxide nanofibrous composite membrane for anionic dyes separation

Development of efficient and durable nanofibrous composite membranes for dyes separation is still a big challenge in the practical applications. Herein, we demonstrated a novel nanofibrous composite membrane consisting of electrospun poly(arylene ether nitrile)(PEN) nanofibrous substrate and bioinspired polydopamine (PDA) coated graphene oxide barrier layer for anionic dyes separation. To achieve this target, PEN nanofibrous mat was prepared by electrospinning technique and hot-pressing treatment. Thereafter, the graphene oxide nanosheets were directly assembled onto above highly porous PEN nanofibrous mat by a simple vacuum suction method. Finally, as-obtained PEN/GO nanofibrous composite membrane was further modified by bioinspired PDA coating, thus forming PEN/GO-PDA nanofibrous composite membrane. Benefiting from the superiority of hydrophilic GO-PDA skin layer and interconnected porous PEN nanofibrous supporting layer, the resultant optimized PEN/GO-PDA nanofibrous composite membrane exhibited an excellent permeate flux of 99.7L/m2h (0.1MPa, pH=3.0) and a high rejection (99.8%) for Direct Blue 14 with the concentration of 100mg/L at 25°C. This novel composite membrane demonstrated excellent mechanical strength of 28.1MPa and achieved high temperature resistance. The rejection of Direct Blue 14 (100mg/L) remained 92.6% and permeate flux reached 141.5L/m2h as the operating temperature increased to 90°C. Moreover, the resultant PEN/GO-PDA nanofibrous composite membrane exhibited good reusability after repeated operations and excellent antifouling properties (the flux recovery ratio reached 95.0% after 3 runs for 9h). This work provides an alternative way to prepare high-performance composite membrane materials, which can deal with multiple challenges for dyes separation.

The Mixed Potential Model for the Passive State and Hydrogen Evolution Reaction of AA2098-T851

Theoretical and experimental investigations for fundamental understanding of the passivation process and its kinetics could lead to the prediction of the electrochemical behavior of the third-generation Al-Li alloy 2098. In this paper, the kinetics of the passive film formation on AA2098-T851 in 0.1M NaHCO3 in a CO2 atmosphere at 25°C, at potentials with both metal dissolution and hydrogen evolution reaction (HER) was investigated. This was done by performing electrochemical impedance spectroscopy (EIS) on the passive layer after each potentiostic steps in the anodic direction that followed by stepping in the opposite direction. The optimization of the EIS data was done by applying the previously developed Mixed Potential Model (MPM) that considers the Point Defect Model (PDM) for dissolution and passivation of the metal along with the Butler-Volmer equation for the cathodic reaction. It was shown that the inequality of certain kinetic processes of passive layer formation leads to the irreversibility of the metallic passivation in opposite potential scanning directions. Also, considering that the hydrogen evolution reaction incorporates quantum mechanical tunneling of the charge carriers through the barrier oxide layer on the passive metal surface, a cathodic Tafel constant (βc ) was calculated. The estimated βc was in accord with the predicted value for the HER according to the slow discharge of protons mechanism.

Redefining the Electrochemical Kinetics of Redox Reactions on Passive Metals

The kinetics of redox reactions are described by specifying a standard exchange current density and a transfer coefficient that locates the position along the reaction coordinate at which the activated complex (transition state) occurs. However, this formulation was originally developed to describe charge transfer reactions on bare metal surfaces. In the case of a passive metal, however, the barrier layer of the passive film represents a barrier to the quantum mechanical tunneling of charge carriers (electrons and electron holes) between the metal and the redox reaction center and theory shows that this barrier has a profound effect upon the tunneling probability and hence upon the values of the exchange current density and the Tafel constant. In this presentation, the role of quantum-mechanical tunneling of charge carriers through the barrier oxide layer on a passive metal, in determining the electrochemical kinetic parameters is explored theoretically. The exchange current density for a redox reaction occurring on the passive metal surface is assumed to be the product of the exchange current density on the bare metal surface and the quantum mechanical tunneling probability (QMTP) of charge carriers through the barrier layer, which is inversely proportional to the exponential of the barrier thickness and the tunneling constant that in turn is proportional to the square root of the product of barrier height and the effective electron mass. The QMTP is used to redefine the exchange current density for the cathodic reaction (e.g., reduction of oxygen), as measured on the passive surface, to that on the (hypothetical) bare metal alone. Thus, knowing the voltage, the barrier layer thickness can be calculated from the Point Defect Model (PDM), which in turn can be used to estimate the QMTP for that particular set of conditions. Then, the exchange current density for the redox reaction on the passive metal surface is calculated and used, if the barrier height and the effective electron mass are constants, in the generalized Butler-Volmer equation to describe the current/voltage characteristics of the redox reaction. In doing so, it becomes evident that the kinetics of a redox reaction are fully determined by specifying a (hypothetical) exchange current density and transfer coefficient of the reaction on the bare metal (“hypothetical” because the bare metal condition may not be realizable experimentally), the QMTP, the electric field strength within the barrier layer, the polarizability of the barrier layer/solution interface, and the potential of zero thickness of the barrier layer. The application of this theory is illustrated with respect to the hydrogen electrode reaction (HER) on passive carbon steel and on aluminum and in predicting the corrosion potentials of stainless steels and nickel alloys in the primary coolant circuits of water-cooled nuclear reactors.

The influence of chromium and molybdenum on the repassivation of nickel‐chromium‐molybdenum alloys in saline solutions

The electrochemical properties and compositions of passive, transpassive, and re‐formed passive oxides on the Ni‐Cr‐Mo alloys C22 (high Cr/low Mo) and HYBRID BC1 (low Cr/high Mo) have been studied in 5.0M NaCl solutions. At low potentials in the passive region, the oxide was less resistive on BC1 compared to C22, possibly due to the influence of Mo in increasing the number of defects in the oxide. At higher potentials in the passive region, both alloys exhibited similar behaviours and composition of oxide films. Under transpassive conditions, the CrIII barrier layer (dominantly Cr2O3) was destroyed on both alloys, with a greater accumulation of MoVI/MoV on the BC1 than on the C22 alloy. Improved passivity was achieved for both alloys on returning to potentials in the passive region. This appears to be due mainly to the re‐formation of the CrIII barrier oxide layer on C22, but to a combination of the reformation of this layer and the accumulation of MoVI/MoV on the BC1. Copyright © 2017 John Wiley & Sons, Ltd.

Characterization of oxide barrier layers prepared by atomic layer deposition

Atomic layer deposition has become an important thin-film growth technique for producing gas diffusion barriers because of its low process temperature and its ability to produce uniform films. In this work, atomic layer deposition was used to deposit various Al2O3 and ZnO thin films on polyethylene terephthalate substrates; subsequently, the physical properties and water vapor transmission rates of the films were characterized. Single and hybrid films (Al2O3, ZnO, Al2O3/ZnO, and ZnO/Al2O3) with thicknesses of 25, 50, and 100 nm at a deposition temperature of 60°C were investigated. The deposited films were characterized for surface roughness, optical transmittance, adhesion, water vapor transmission rate, and contact angle. The results showed that the double-layer structure provided a higher water vapor transmission rate and higher adhesion strength than those of the single-layer structure although both the surface roughness and optical transmittance of the single-layer structure were slightly better than those of the double-layer structure. The results revealed that the atomic layer deposition-grown hybrids could act as water vapor barriers.

Determining the Conduction Band-Edge Potential of Solar-Cell-Relevant Nb2O5 Fabricated by Atomic Layer Deposition.

Often key to boosting photovoltages in photoelectrochemical and related solar-energy-conversion devices is the preferential slowing of rates of charge recombination-especially recombination at semiconductor/solution, semiconductor/polymer, or semiconductor/perovskite interfaces. In devices featuring TiO2 as the semiconducting component, a common approach to slowing recombination is to install an ultrathin metal oxide barrier layer or trap-passivating layer atop the semiconductor, with the needed layer often being formed via atomic layer deposition (ALD). A particularly promising barrier layer material is Nb2O5. Its conduction-band-edge potential ECB is low enough that charge injection from an adsorbed molecular, polymeric, or solid-state light absorber and into the semiconductor can still occur, but high enough that charge recombination is inhibited. While a few measurements of ECB have been reported for conventionally synthesized, bulk Nb2O5, none have been described for ALD-fabricated versions. Here, we specifically determine the conduction-band-edge energy of ALD-fabricated Nb2O5 relative to that of TiO2. We find that, while the value for ALD-Nb2O5 is indeed higher than that for TiO2, the difference is less than anticipated based on measurements of conventionally synthesized Nb2O5 and is dependent on the thermal history of the material. The implications of the findings for optimization of competing interfacial rate processes, and therefore photovoltages, are briefly discussed.

Bi-functional anodic TiO2 oxide: Nanotubes for wettability control and barrier oxide for uniform coloring

A uniformly colored TiO2, on which the surface is functionalized with nanotubes to control wettability, was prepared by a two-step anodization; the first anodization was carried out to prepare nanotubes for a super-hydrophilic or -hydrophobic surface and the second anodization was performed to fabricate a thin film barrier oxide to ensure uniform coloring. The effect of the nanotubes on barrier oxide coloring was examined by spectrophotometry and UV-vis-IR spectroscopy. We found four different regimes governing the color changes in terms of anodization voltage, indicating that the color of the duplex TiO2 was primarily determined by the thickness of the barrier oxide layer formed during the second anodization step. The surface wettability, as confirmed by the water contact angle, revealed that the single barrier TiO2 yielded 74.6°±2.1, whereas the nanotubes on the barrier oxide imparted super-hydrophilic properties as a result of increasing surface roughness as well as imparting a higher hydrophobicity after organic acid treatment.

Layered microstructures based on BaZr0.85Y0.15O3−δ by pulsed laser deposition for metal-supported proton ceramic electrolyser cells

Planar metal-supported cell designs provide cost-effective scaling-up of solid oxide fuel cells and electrolysers. Here, we report on the fabrication of a BaZr0.85Y0.15O3−δ–NiO (BZY15–NiO) composite electrode and BaZr0.85Y0.15O3−δ (BZY15) proton-conducting electrolyte films on metal and ceramic substrates using pulsed laser deposition (PLD). The results demonstrate successful sequential deposition of porous electrode and dense electrolyte structure by PLD at moderate temperatures, without the need for subsequent high-temperature sintering. The decrease in roughness of the metal substrate used for deposition by spray-coating intermediary oxide layers had significant importance to the fabrication of functional layers as thin films. Crystalline porous BZY15–NiO and dense BZY15 films were sequentially deposited at high substrate temperature on metal supports (MS) with or without an electron-conducting barrier oxide layer, e.g. MS/(BZY15–Ni)/(BZY15–NiO)/BZY15 and MS/CeO2/(BZY15–NiO)/BZY15. The different microstructures for electrode and electrolyte were achieved with deposition steps at different substrate temperatures (800, 600 °C) and a gradual decrease in the pressure of O2 in the deposition chamber.

TaNX coatings deposited by HPPMS on SS316L bipolar plates for polymer electrolyte membrane fuel cells: Correlation between corrosion current, contact resistance and barrier oxide film formation

Tantalum nitride (TaNx) coatings deposited by High Power Pulsed Magnetron Sputtering (HPPMS) technology at different N2-to-Ar ratios; i.e. 0, 0.25, 0.625 and 1, corresponding to different N/Ta atomic ratio in the film, were investigated as suitable candidates to improve SS316L bipolar plate's performance and durability for polymer electrolyte membrane fuel cells (PEMFC). The corrosion resistance of TaNx coatings was evaluated by potentiodynamic and potentiostatic tests carried out under different cathodic potentials (0.8 VSHE, 1 VSHE, and 1.4 VSHE), electrolyte acidities (pH 3 and pH 6) and test durations (5 and 180 min) in order to mimic real fuel cell operation conditions. Corrosion currents observed for all TaNx coatings were relatively low (1–15 μA cm−2) regardless of N/Ta atomic ratio and applied variable testing parameters. However, considerable differences in Interfacial Contact Resistance (ICR) values were observed after polarization, depending on tested coating material and conditions. The ICR increased with increasing applied potential, electrolyte pH and test duration for the substrate and all TaNx coatings. Ta coated SS316L exhibited lower ICR (42–82 mΩ cm2) values than the uncoated SS316L (47–278 mΩ cm2) at potentials higher than 1 VSHE. A significant rise in ICR was detected for all nitride TaN films after 180 min of polarization at 1.4 VSHE in pH 3, showing ICR values from 362 to 538 mΩ cm2, depending on N2-to-Ar ratio. Ta coated SS316L polarized at 0.8 VSHE showed low ICR values, around 25–37 mΩ cm2. Auger electron spectroscopy (AES) was performed before and after polarization to investigate barrier oxide film formation kinetics. AES study revealed the growth of different composition and thickness oxide layers for each TaNx coating, exposing the great importance of coating composition on subsequent type of oxide formation. Barrier oxide layer characteristics have been found to dominate the ICR response of TaNx films after polarization.

Robust construction of a graphene oxide barrier layer on a nanofibrous substrate assisted by the flexible poly(vinylalcohol) for efficient pervaporation desalination

A novel thin-film nanofibrous composite (TFNC) membrane with a robust graphene oxide barrier layer assisted by PVA for efficient pervaporation desalination.

Photo- and electroluminescence properties of lanthanide tungstate-doped porous anodic aluminum oxide

A new cathode material for the potential use in light-emitting devices, based on porous anodic alumina (PAA), aluminum and ITO layers has been synthesized. Porous alumina samples with ordered pore arrays were prepared electrochemically from high purity Al sheet in H2SO4 and H3PO4. To be able to apply the matrix obtained in the electroluminescence cell, the thickness of the barrier layer of aluminum oxide was decreased by slow reduction of the anodization voltage to zero. The luminescence and electroluminescence (EL) properties of the Al2O3 matrix admixtured with Eu3+ and Tb3+ ions as well as europium and terbium tungstates, were determined. The particles of inorganic luminophore were synthesized on the walls of the matrix cylindrical nanopores in the two-step process of immersion in solutions of TbCl3 or EuCl3 and Na2WO4. The effect of the nanopores diameter and the thickness of the porous Al2O3 layer on the intensity and relative yield of electroluminescence was analyzed, the best results were obtained for 80–90 µm PAA layers with 140 nm nanopores.

A 2D and 3D X-ray μ-diffraction and μ-fluorescence study of a mixed ionic electronic conductor

Due to the mixed ionic electronic conductive properties of the Lanthanum Strontium Cobalt Ferrite (LSCF) La0.6Sr0.4Co0.2Fe0.8O3−δ compound, it is of great scientific and technological interest. Especially in the Solid Oxide Fuel Cell (SOFC) technology, this compound has receiving great attention as a cathode material. However, its chemical reactivity with the Yttria-stabilized Zirconia (YSZ) electrolyte still remains one of the main challenges, which demands a comprehension in the μm and sub-μm range. In order to address the reactivity issues locally in the micrometre scale range, 2D and 3D X-ray μ-diffraction and μ-fluorescence analysis have been performed on a pristine LSCF cathode layer. The cathode was deposited on a dense YSZ electrolyte substrate spaced by a thin Gadolinium doped Ceria Oxide (CGO) barrier layer in between LSCF and YSZ to limit the reactivity. The present approach offers a larger field of view in comparison to electron microscopy techniques. The method can provide a more representative information and may offer some insights on the reactivity distribution along the interfaces. The formation of micro SrZrO3 inclusions in LSCF layer is then indubitably identified, as well as in the CGO/YSZ interface.

A Comparative Study of Anodic Aluminum Oxide Preparation for Synthesis of Nanomaterials

A template-based synthesis of nanomaterials is one of the most simple and low-cost fabricating way. The template-assisted method, which employs porous anodic aluminum oxide (AAO) templates, allows for a precise control of the morphology and dimensions of deposited nanowires [1, 2]. Nanoporous Al2O3 can be prepared by a two-step anodization of aluminum in acidic electrolytes. During the anodization, Al foil [2] or Al film sputtered on conducting substrates [3] is subjected to anodic oxidation. The anodization of Al foil results in the Al2O3/Al substrate, which is not suitable to carry out a direct electrodeposition into the pores, due to the presence of the non-conductive oxide barrier layer at the oxide/metal interface. To enable a direct electrodeposition in the Al2O3/Al substrate, the barrier layer at the pore bottom has to be removed. There are two basic methods used for the removal of the barrier layer: wet chemical etching [1, 2] and electrochemical [4–8]. The first one, the wet chemical etching requires additional steps, which include: dissolving the aluminum layer remaining after anodization of the Al foil, pore opening and widening in a phosphoric acid solution to obtain a through-hole membrane, and the sputter deposition of the conductive metal layer that serves as a working electrode during electrodeposition. Drawbacks of this method are: the use of harmful chemicals (HgCl2, CuCl2) [2] or concentrated acid solutions (H3PO4, HCl) [1, 2], and multi-stage process (loss of the material at each stage). The alternative method is based on the electrochemical reduction of the barrier layer. Due to the fact that the thickness of the barrier layer depends on the anodizing potential, a gradual decrease in the applied potential leads to a significant reduction of the barrier layer thickness. As a result of this treatment we obtain AAO, with dendritic channels with a very thin barrier layer at the pore bottoms. Such a structure allows to conduct the direct electrodeposition on the aluminum substrate without any additional steps. An important advantage of this method is the fact that the potential reduction is carried out in the same electrolyte in which the anodization was realized. Therefore, the potential reduction can be performed immediately after the anodizing process. On the other hand, there is a new electrochemical method developed by Lira and Paterson [7] for AAO detachment from the aluminum substrate. It was found that a large voltage pulse applied to the sample results in simultaneous membrane detachment and pore opening. The voltage detachment method has significant advantages over the typically used chemical etching, e.g., it is faster, and environmental friendly. As-obtained AAO membranes are suitable for electrodeposition of metallic nanostructures. Herein, we report on investigation of electrochemical methods for the AAO membrane preparation and their optimization toward further metallic nanostructure deposition. Highly ordered nanoporous AAO templates were fabricated by a two-step self-organized anodization of aluminum carried out in oxalic and sulfuric acids solutions. In the first method, the barrier layer thinning was done by applying an exponentially decreasing voltage at the end of anodization process. Depending on the function used for the reduction of the barrier layer and its thickness, the dendrite-like structures of the barrier layer were observed at the cross-sections. In the second method, a modified voltage detachment method in a mixture of HClO4 and ethanol was used. Different pulses were applied for the detachment of alumina formed in oxalic and sulfuric acids.

(Invited) Study of Barrier Oxide Growth on Impure Aluminum in Aqueous Solution By Chronoamperometry and Electrochemical Impedance Spectroscopy

The growth of the barrier oxide layer on aluminum during low-current anodic polarization in aqueous solutions occurs in accordance with the high-field Cabrera-Mott growth law,1 as recently confirmed for pure aluminum by use of simultaneous ellipsometry and chronoamperometry to measure the transient film growth following a potential step.2 Electrochemical impedance spectroscopy (EIS) was used to determine the steady-state barrier layer thickness between each potential step as in situ calibration for the measurement of the film growth rate by the other two methods. Complementarity of the three techniques was demonstrated for studying the kinetics of growth and dissolution of thin passivating oxides in aqueous solution. The purpose of the present study is to investigate whether the same approach can be used for studying the modification of the interface by contamination by the common trace element Pb as a result of heat treatment. Pb is known to activate the surface by destabilizing the passivating barrier layer.3 The microstructure and composition of the contaminated interface has been characterized by ex-situ STEM and various surface analytical techniques for samples heat treated at high (600°C) temperatures, resulting in detectable Pb segregation in the form of both nanosize (up to 10 nm) Pb particles and a few nm thick Pb-rich film along the metal-oxide interface. Pb segregation by heat treatment at lower temperatures was not possible by these techniques. Supplementary data were necessary for samples heat treated at lower temperatures, especially for modification of the electrochemical properties of the interface. The samples investigated were model AlPb alloys prepared from pure components in the laboratory and cold rolled into 2.2 mm thick plates, followed by polishing metallographically through 1 µm surface finish. They were subsequently heat treated in an air-circulation furnace in the temperature range 300 - 500°C. The amorphous oxide is reported to thicken moderately in the range 10 - 20 nm as a result of 1 h heat treatment at this temperature range.4 Experiments were conducted in a conventional three electrode electrochemical cell in the manner described above. Ellipsometry could not be used because of slow measurement relative to the rate of barrier-layer growth and unexpected loss of specularity of the sample surface during heat treatment. The EIS data were evaluated by use of the equivalent-circuit model derived rigorously by Armstrong and Edmondson,5 which was shown to be applicable to the present electrochemical interface in the case of pure Al.2 Oxide growth was calculated from the current-time data by use of Faraday's law corrected for current leakage (passive current). The growth data were analyzed by use of the integrated form of the Cabrera-Mott law, according to Ghez,6 giving the well-known inverse logarithmic growth law. The changes in the barrier layer thickness appeared to fit the Ghez equation, but not as successfully as pure Al. The EIS data showed that the film thickened with increasing applied potential at the rate of 1.2 nm/V for all heat treatment temperatures except for 600°C, as expected for pure Al. The passive current density increased and the film resistance decreased at a given applied potential, with increasing annealing temperature, indicating reduced passivity. However, the film capacitance, measured by EIS at a given applied potential, increased with annealing temperature, giving an apparent decrease in the film thickness with increasing annealing temperature, in contrast to the expected trend to the opposite.4 The changes in the barrier film properties, measured by EIS, are attributed to the segregation of metallic Pb at the metal-film interface, in the film, and possibly at the film surface, contributing to the increased capacitance of the electrochemical interface. The model used for correlation of the EIS data has to be modified to incorporate the effect of segregated Pb in the form of particles and continuous film for more reliable measurement of the barrier film thickness in future work. References J. W. Diggle, T. C. Downie, and C. W. Goulding, Chem. Rev., 69 (3), 365 (1969). N. H. Giskeødegård, O. Hunderi and K. Nisancioglu, J. Solid State Electrochem. 19, 3473 (2015). Anawati, B. Graver, H. Nordmark, Z. Zhao, G. S. Frankel, J. C. Walmsley, and K. Nisancioglu, J. Electrochem. Soc., 157, C313 (2010). E. Senel, Dissertation, Norwegian University of Science and Technology, Trondheim (2013). R. D. Armstrong and K. Edmondson, Electrochim. Acta, 18,937 (1973). R. Ghez, J. Phys. Chem., 58, 1838 (1973).

Fabrication of highly ordered short free-standing titania nanotubes

Highly ordered TiO2 nanotube arrays were fabricated by anodic oxidation of titanium thin film in organic electrolyte containing ammonium fluoride and water. The effect of electrolyte composition as well as sputter deposition parameters of titanium thin film on morphology of nanotubes was observed by scanning electron microscopy (SEM) and surface chemical analysis by X-ray photoelectron spectroscopy (XPS). The electrolyte composition was optimized to reach the balance between anodic oxidation and chemical etching of initial oxide barrier layer, leading to freestanding and highly ordered open nanotubes with lengths in the range of 400–800 nm. Further these surfaces of nanostructured titania may be easily decorated with broad spectrum of nanoparticles or various functional materials according to required applications.